May 28, 1998

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

Earth Tides and Volcano Monitoring

The gravitational attraction of the Sun and the Moon produce the familiar ocean tides and the less
familiar earth tides. Why are volcanologists interested in earth tides? Earth tides are cyclical, small,
and slow ground movements that we use to calibrate and test sensitive volcano deformation-
monitoring instruments. They might also trigger volcanic events.

The tides are slight bulges of the ocean's or Earth's surface that face the Moon and the Sun as the
Earth rotates on its axis. There are actually two lunar and two solar tidal bulges, one on the closest
and the other on the farthest side of the Earth. The lunar bulges are a little more than twice the
height of the solar bulges. At new and full moon, the Sun and the Moon are aligned, and the lunar
and the solar bulges add together for the greatest tidal range. At first- and third-quarter phases of
the moon, lunar and solar tides are in opposition, and the tidal range is at a minimum. The tides go
through one full cycle (a high and low tide) about once every 12 hours and one full cycle of
maximum height (a spring and neap tide) about once every 14 days.

When there is a large earthquake or when you are near a source of volcanic tremor, you can feel the
ground move. At other times volcanic ground movements are so small or slow that they can only
be measured by seismometers, tiltmeters, strainmeters, or other sensitive geophysical sensors.
Movements of magma occur over periods from minutes to days, outside the detection range of
most seismometers.

The slow ground movements associated with magma moving underground can be detected with
tiltmeters and strainmeters. We install these instruments deep in the ground to isolate them from
the daily and yearly heating and cooling of the ground. How do we test whether they are working?

The periods of the earth tides are in the same range as those associated with magma movements,
and they provide a critical test for the response of the deformation instruments. The earth tides are
quite small and can only be measured with the most sensitive instruments, such as those deployed
by HVO.

The forces that produce the tides are a tiny fraction of the forces that cause earthquakes and
eruptions. Even though the forces are small, they can trigger an event. Scientists have found no
correlation between the tides and earthquakes. Correlations between the tides and eruptions,
however, have been identified.

Most people are not even aware that there are earth tides. These slow and rhythmic undulations of
the Earth's surface are used by volcanologists to calibrate and test sensitive deformation-
monitoring instruments. They can also trigger volcanic eruptions.

Eruption and Earthquake Update

Kilauea's east rift zone eruption continues, with lava traveling in tubes from the Pu`u `O`o area to
the coast. Last week there was a brief pause in eruptive activity, lasting from midday on May 19
until the early morning hours of May 21. When the eruption resumed, numerous surface flows
broke out of the reoccupied tube from the top of Pulama pali down to the coast. Lava reentered the
ocean at the existing Kamokuna entry point shortly after noon on May 21. By Monday, May 25,
only a few surface flows on the coastal plain were still active, and both the Kamokuna and
Wahaula entries were active.

There were no felt earthquakes last week.

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Contact:
hvowebmaster@usgs.govUpdated:
29 May 1998